Cleavage - Unequal Fourth Cleavage

Although sea urchin embryos undergo radial cleavage, the cleavages are
decidely unequal at the fourth cleavage, when the vegetal tier of blastomeres
undergoes equatorial cleavages top produce four Micromeres at the extreme
vegetal pole, and four large macromeres.

16-cell L. variegatus zygote, viewed from the side
(left) and from the vegetal pole (right). Micromeres form at the vegetal
pole, macromeres form immediately above them, and eight equal-sized mesomeres
form at the animal pole.

The vegetal blastomeres are apparently already "programmed"
to do this, as they will undergo unequal cleavage after being isolated at
the 8-cell stage.

Isolated vegetal and animal blastomeres from an 8-cell embryo. Can you
guess which one is the animal blastomere and which one is the vegetal?